This invention relates to variable reluctance actuators, particularly variable reluctance actuators whose mechanical force may be controlled throughout the range of movement of their movable actuation element.
Variable reluctance actuators operate on the principle that a magnetic material, when placed in a magnetic field, will experience a mechanical force tending to move the material in a direction parallel to the field, the mechanical force at any point on the surface of the material being proportional to the square of the flux density of the magnetic field experienced at that point. A magnetic material is a material that exhibits enhanced magnetization when placed in a magnetic field.
In a practical variable reluctance actuator a movable element made of magnetic material, typically in the form of a ferromagnetic plunger, is subjected to a magnetic field generated by an electrical current in a coil so that it transmits the resultant force to some other device for actuation. Such an actuator is referred to as a "variable reluctance" actuator because as the movable element, which makes up part of a magnetic circuit, moves in response to mechanical force, it varies the reluctance within the magnetic circuit, ordinarily by changing the dimension of an air gap.
A typical example of a variable reluctance actuator is a linear actuator comprising a plunger mounted for sliding inside the core of a solenoid. (Although the term "solenoid" is loosely used commonly to refer to such a device as a whole, it is used herein in its technical sense to refer to a coil comprising one or more layers of windings of an electrical conductor ordinarily wound as a helix with a small pitch.) Such linear actuators are used, for example, in vehicles, household appliances, and a variety of industrial applications, such as for controlling valves.
Variable reluctance actuators are to be distinguished from actuators in which mechanical force is created as a result of current passing through a conductor oriented perpendicular to a magnetic field, thereby creating lateral force on the conductor, the conductor typically being wound in the form of a movable solenoid. In general, the latter are more difficult to construct and provide less actuation force per unit volume.
A principal problem with variable reluctance actuators which limits the applications to which they may be put is that the mechanical force experienced by the moving element in the actuator changes as a function of the position of the moving element. Ordinarily the change is non-linear, the force increasing more rapidly as the effective air gap in the device decreases, since the decrease in air gap produces a decrease in circuit reluctance and a concomitant increase in circuit flux. This generally causes the moving element to release energy in the form of undesirable vibration and noise when it collides with a stop for limiting its excursion, and makes controlled positioning of the element difficult. While the force can theoretically be controlled by controlling the current in the solenoid this has heretofore been difficult to accomplish effectively. Consequently, such devices are ordinarily used in simple on-off applications where the vibration and noise resulting from collision of the moving element with a stop is of little or no significance, and are often relatively crude devices.
Previous approaches to controlling the mechanical force created by variable reluctance actuators so as to employ them in more sophisticated applications have employed transducers to measure the mechanical force or the position of the moving element to provide feedback for controlling the current in a solenoid. One example of such an approach is shown by Keller U.S. Pat. No. 3,584,496 wherein a force-sensitive transducer is employed to measure the mechanical force applied by the moving element of a variable reluctance actuator and the output of the transducer is employed to control the current in the solenoid of the actuator. In Umbaugh U.S. Pat. No. 3,697,837, the position of the moving element is also detected by a displacement-sensitive transducer to control the current in a solenoid and, hence, the position of the moving element.
Some drawbacks of measuring the actual mechanical force experienced by the moving element, which requires a device sensitive to change in physical dimensions, such as a strain gauge, are that such devices are typically sensitive to orientation, inertia, and shock, have slow response times, and require complex circuits to control the current in the magnetic field generating coil. While devices for measurement of the position of the moving element can be more readily employed to adjust the position of the moving element, they are subject to some of the same problems. Moreover, they cannot be used to adjust the mechanical force without knowledge of, and compensation for, the force-position characteristic of the actuator.
Since the force experienced by the moving element of a variable reluctance actuator is proportional to the square of the magnetic flux density experienced by the element, it would be desirable to measure that magnetic flux density directly. Although coils have been used to detect a change in magnetic field strength in bi-stable variable reluctance actuators, as in Massie U.S. Pat. No. 3,932,792, a coil cannot be used to measure the instantaneous magnitude of magnetic field strength, or flux density. An alternative would be to use a Hall effect device, whose output is a function of the magnetic flux density that it experiences. While Hall effect devices have been used in connection with permanent magnets as position detectors, as in Brace et al., U.S. Pat. No. 4,319,236, it is believed that they have not been used to measure the flux density experienced by a moving element in a variable reluctance actuator.
lt would also be desirable to control the flux density in the moving element of a variable reluctance actuator by controlling the duty cycle of the solenoid in order to maximize energy efficiency. Electronic circuits for switching the current in a solenoid on and off in a varible reluctance actuator, including a flyback diode for protecting the circuitry from unacceptable voltage excursions during the collapse of the magnetic field in the solenoid, have been used, for example, in electronically driven pumps, as shown in Maier et al., U.S. Pat. No. 3,293,516; however, such devices are bistable, and do not control the current in the solenoid to maintain substantially constant flux density in the moving element.
In addition, it would be desirable to control the position of the moving element of a variable reluctance actuator based upon the magnetic and electrical characteristics of the actuator itself, rather than an external transducer subject to difficult-to-control variables.